4. Answer eight of the following (a), (b), (c), etc - Leaving Cert Chemistry - Question 4 - 2015

To determine the number of electrons and neutrons in the $ext{Se}^{2-}$ ion:

1. Electrons: A neutral selenium (Se) atom has an atomic number of 34, meaning it has 34 protons and, therefore, 34 electrons. Since the ion is $ext{Se}^{2-}$, it has gained 2 additional electrons, resulting in a total of: $34 + 2 = 36\text{ electrons}.$

2. Neutrons: The isotope of selenium represented typically weighs about 78 (the most stable isotope is $ext{Se}^{78}$). The number of neutrons is determined by: $\text{Neutrons} = \text{mass number} - \text{atomic number} = 78 - 34 = 44\text{ neutrons}.$